1. Field of Invention
This invention relates to a transfer system used for transfer of wafers in a semiconductor manufacturing line, or the like.
2. Description of the Prior Art
In the manufacture of semiconductors, for example, a transfer system is used to move an object being processed, such as a semiconductor wafer, from one process to another process. Examples of conventional transfer systems are discussed below.
FIG. 1 shows a first example of a conventional transfer system, wherein two upper arms 11,12 are positioned on a drive shaft 13. Drive shaft 13 is designed to be driven by rotation. Upper arms 11,12 are linked to drive shafts 14,15. Upper arms 11,12 rotate on drive shaft 13. To the tip of upper arm 11, forearms 16,17 are rotatably mounted. To the tip of upper arm 12, forearms 18,19 are rotatably mounted. Platform 20 is linked to the tips of forearms 16,18. Platform 21 is linked to the tips of forearms 17,19. The four forearms 16-19 are assembled to form a pair of frog-legs like structure. Platforms 20,21 are positioned depending on the rotated angles of upper arms 11,12.
Operation of the system of FIG. 1 is as follows. By turning two upper arms 11,12 in opposite directions, one platform (20 or 21) moves in a direction away from drive shaft 13 (i.e. the distance from drive shaft 13 is increased) and the other platform (21 or 20) is moved only slightly from its standby position. The other platform stays approximately at the same position as the standby position, wherein the standby position is a position disposed above drive shaft 13.
In addition, by rotating drive shaft 13, the orientation of the frog-legs like structure is changed. This makes the extending direction of platforms 20 and 21 change. From these operations, two wafers are transferred by (a) selecting either of platforms 20 and 21, and (b) then extending or contracting that platform which is selected depending on the angle formed with the upper arms 11 and 12. Drive shaft 13 is rotated when a wafer, upon which processing is completed, is removed from the process using a platform and then is transferred to another process.
However, the conventional system of FIG. 1 has certain problems. For example, if the direction of extension of platforms 20,21 is to be changed, drive shaft 13 is rotated with the platforms 20,21 being placed in the standby position. In order to ensure that platforms 20, 21 do not contact adjacent machines, equipment, etc, the turning radius of the platforms 20,21 must be small. In that case, platforms 20,21 must be made to move close to drive shaft 13, which limits the thickness of the forearms 16-19. Hence, the forearms must be made to be very thin, and suitable rigidity cannot be achieved. Accordingly, due to the weight of the wafers and platforms, the forearms are subjected to bending, and stable transfer is prevented from occurring and speed of transfer cannot be increased.
FIG. 2 shows a second conventional transfer system wherein first arm 32 is arranged rotatably around rotating shaft 31; second arm 34 is arranged rotatably around rotating shaft 33 located on the tip of first arm 32; and third arm 36 is arranged rotatably around rotating arm 35 located on the tip of second arm 34. The center of rotation of third arm 36 is at the midpoint thereof. First arm 32 is rotated by being directly coupled to a first motor (not shown). Second arm 34 and third arm 36 are rotated by a second motor and a third motor (not shown) via pulleys and belts. The second conventional transfer system has Selective Compliance Assembly Robot Arm (also called xe2x80x9cSCARABxe2x80x9d) types of arms. The test object mounting parts 37,38, on which test objects are disposed, respectively, are provided at both ends of third arm 36.
Operation of the FIG. 2 system is as follows. The stand by position is a position whereat first arm 32 and second arm 34 overlap each other. Rotation of first arm 32 in one direction causes test object mounting part 37 or 38 disposed on either tip of third arm 36 to be placed in an extended position which is furthest from rotating shaft 31. The other test object mounting part, located on the opposite side, is positioned in a place nearer to the rotating shaft 31 than the test object mounting part located furthest away. Rotation of first arm 32 in the opposite direction moves the test object mounting part on the opposite side to the furthest extended position via the stand by position. In addition, the extending direction of a test object mounting part is set at an arbitrary angle by turning the system around rotating shaft 31. Accordingly, two wafers are transferred to other places by moving the two test object (e.g. wafers) mounting parts to the standby position or the furthest extended position.
However, the second conventional transfer system has the following problems. First arm 32 and second arm 34 are provided with a sealed barrier to exclude particles generated or provided by wear of the bearings that support the belts and pulleys used therein, in order to obtain the degree of cleanliness required for semiconductor manufacture. Also, the width of the belts depend on the strength required for transmitting power. Accordingly, thicker arms must be used to insure provision of the required width of the belts, and to provide barrier construction.
Moreover, in SEMI E21 and SEMI E22 standards, which are the standards generally followed in the industry to manufacture semiconductors, standard dimensions of the arms are provided. Specifically, the thickness of the hands in the interface zones for engaging test objects is defined to be 23 mm for wafers up to 8 inches; that is, a very thin dimension is required by the industry standards. In the FIG. 2 system, the coupling part of the second arm 34 and third arm 36 correspond to this zone. Making the thickness of the coupling part to be 23 mm or less and providing barriers incorporating the belts and pulleys and bearings in the arms, result in fragile construction of the arms. Thus, in the prior art, the required arm thickness cannot be satisfied for such arm construction. Moreover, the arms are likely to become bent due to the weight of the wafers and the test object mounting parts, because the arms must be designed to be thin. Thus, stable transfer of wafers from one process to another cannot be assured.
FIG. 3 shows a third conventional transfer system, wherein two forearms 43,44 are rotatably mounted on the tips of rear arms 41,42. Transfer base 45 is linked to the tips of forearms 43,44 using hinges 46,47. The structure resulting from such construction is a frog-legs shaped structure. Rear arms 41,42 are rotated by gears 48,49 that rotate in opposite directions to each other and in synchronism. Forearms 43,44 are each rotated by pulleys 50,51 having an effective diameter ratio of 2 to 1, and tension belt 52 is stretched between both pulleys 50,51. Pulley 50 is tightly fixed in a coaxial manner to gears 48,49. Pulley 51 is tightly fixed in a coaxial manner to hinges 53,54.
Operation of the FIG. 3 system is as follows. Forearms 43,44 are rotated to an angle 2xcex1, which is twice the deflection angle xcex1 of each of rear arms 41,42 when the rear arms are rotated in opposite directions to each other and transfer base 45 is positioned. The rotating angles and rotating directions of the forearms, which are restricted by pulleys 50,51 and belt 52, correspond to the rotating positions of the rear arms. Hence, transfer base 45 is positioned by the rotation of each rear arm in the range of xc2x190xc2x0 in opposite directions starting at the condition where each of the forearms and rear arms overlap.
The FIG. 3 system has the following problems, however. The arm construction is equivalent to the case where two SCARA type robots are placed in parallel. Thus, it is a mere addition of a power transmitter such as gears 48,49 that cause rear arms 41,42 to turn in opposite directions and use the arm driving motor commonly.
The two rear arms must be arranged to be close to each other and hence thick gears 48,49 cannot be used. Hence, bearing rigidity is lowered, and the arms are thin. Accordingly, the arms are likely to become bent and stable transfer is not possible. Also, if wafers are transferred in vacuum, the drive shaft must be vacuum sealed also. However, if the gears 48,49 linking the two rear arms 41,42 are required to be independent of each other, each must be separately sealed. Thus, additional maintenance time and labor would be required.
Moreover, considering the relationship between the arms and the motors, to move the arrangement either in the arm extending direction (called xe2x80x9cR-axisxe2x80x9d direction) or in the arm turning direction (called xe2x80x9cxcex8-axisxe2x80x9d direction), the foregoing systems are provided with an R-axis motor and a xcex8-axis motor. When extending or engaging in the construction, one of the motor is operated while the other motor is not operated. Accordingly, availability of the motors leaves much to be desired.
Accordingly, an object of the invention is to overcome the aforementioned and other problems, disadvantages and deficiencies of the prior art.
Another object is to provide a transfer system which provides stable transfer operation and wherein maintenance is simplified and energy consumption is reduced.